Magnetic electron lens



Jan. 15, 1957 J. B. LE POOLE MAGNETIC ELECTRON LENS 2 Sheets-Sheet 1Filed Jan. 14, 1952 INVENTOR Jon Barf le Poole y WW Ag nt Jan. 15, 1957J. B. LE POOLE 2,777,958

MAGNETIC ELECTRON LENS Filed Jan. 14, 1952 2 Sheets-Sheet 2 Fig. 3a

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I [I H I 28 INVENTOR. Fig. 40 JAN BART LE POOLE GENT United StatesPatent MAGNETIC ELECTRON LENS Jan Bart Le Poole, Delft, Netherlands,assignor to Hartford National Bank and Trust Company, Hartford, Conn.,as trustee Application January 14, 1952, Serial No. 266,298

Claims priority, application Netherlands February 10, 1951 7 Claims.(Cl. 250-495) An electron beam can be circularly deflected by ahomogeneous magnetic field, of which the lines of force are at rightangles to the beam. It is known that such a field has a focussing effecton the electron beam in a radial direction. This effect is utilised incertain electron-spectrometers.

According to the invention, a transverse field magnetic lens forelectrons or other energized particles, is characterised in that one ofthe two pole surfaces is a sector having an angle (lens angle) of notmore than 270 of a plane of revolution and in that a plane at rightangles to the axis of revolution, intersecting with one of the tangentsof the generatrix of the plane of revolution under an angle of 30 orless at a distance from the axis of revolution which is equal to that ofthe tangent point, is the plane with respect to which the second polesurface is the reflection of the first pole surface or else forms thecounterpole surface itself. The counterpole surface is defined to mean apole surface located either along the plane which is perpendicular tothe axis of revolution or located to constitute a mirror image of thefirst pole surface with respect to that plane, the two surfaces definingthe region through which the charged beam passes and is acted upon bythe magnetic field existing between the two surfaces.

As will be seen from the further explanation, a lens of particularquality is obtained, if the generatrix of the plane of revolution formspart of a parabola, the apex of which lies in the axis of revolution.Each tangent of this curve intersects the equatorial plane at a pointbeing equally spaced apart from the axis of revolution as the tangentpoint.

However, with practically sufficient approximation, 21 parabola may bereplaced by a tangent which gives the plane of revolution the shape of aconical surface. The tangent which, as far as its inclination withrespect to the equalatorial plane and its point of intersection withthis plane are concerned, fulfills the aforesaid condition,

coincides in this case with the generatrix.

The optical axis of the lens according to the invention is a circlesection, of which the center lies on the axis of revolution and of whichthe radius varies with the magnetic field strength and the velocity ofthe particles. With a lens having conical pole surfaces the size of thisradius acts upon the astigmatism of the lens. It is in this case aminimum, if the spacing between the pole surfaces in the area of theoptical axis is twice the spacing between their point of intersectionwith the axis of revolution (the cone tops).

The strength of the lens is determined, among other things, by the sizeof the lens angle. Suitable values for this angle to ensure a stronglens lie between 120 and 135. The maximum lens strength is obtained atan angle of 127% It is usually inefficient to energise a magneticelectron lens, of which the lines of force extend primarily in thedirection of the electron paths (longitudinal field lens) by means of apermanent magnet. This would be useful, if the electrons should have avelocity which is materially lower than that usually employed inelectron microscopes. Since the transverse field lens, owing to thefocussing efiect, requires a materially smaller magneto-motive force,the energisation is effected here by permanent magnetism, even atconsiderably higher electron velocities. This implies a simplificationof the construction (no energising current circuit, no cooling) and aneconomy in losses.

The optically operating field of the electron lens must be arranged in avacuous space. The energising winding of the radially symmetricallongitudinal field lens can be readily arranged outside the vacuousspace, which is surrounded by the cylindrical yolre. The shape of thetransverse field lens is less suitable for a similar arrangement. lt'iseasier to arrange this lens as a whole in the vacuous space. Thepermanent magnet eliminates the disadvantage involved in the arrangementof an energising winding in the vacuous space.

Since the radius of curvature of the optical axis of the transversefield lens varies with the velocity of the energised particles, theimage produced by this lens shifts in place at a variation of theacceleration voltage. This property of the transverse field lens may beutilised to scan the image; however, the same property involves adisadvantage, when the transverse field lens is used as anelectron-optical system in an electron microscope, of which the workingvoltage is not sufficiently constant. This disadvantage may be mitigatedby providing the microscope with a system of two or more transversefield lenses, which are traversed in succession by the electron beam andwhich deflect the latter in the same plane and in the same sense. Theselenses may be arranged relative to one another and to the object in amanner such that the resultant image produced by the lens system doesnot exhibit any or substantially any chromatic shift.

In a particularly arranged electron microscope according to theinvention the electron-optical lens system directs the electron beam onthe mirror formed by the electrodes of the electron gun and reproducingon an amplified scale the image on which the beam thus directioned isfocussed.

It is efiicient to interconnect the pole shoes of two or more transversefield lenses by means of a common yoke. In this case the co-opera-tinglenses constitute a structural unit.

In order that the invention may be more clearly understood and readilycarried into effect, it will now be described more fully with referenceto the accompanying drawing by way of example.

Fig. 1 serves to explain the focussing effect of a homogeneous magneticfield on electron paths.

Figure 2 shows a pole shoe of a transverse field lens viewed in thedirection of the axis of revolution.

Fig. 3 is a cross sectional view of the transverse field lens, withwhich the pole shoe shown in Fig. 2 is associated, taken in the planeIIIIII of Fig. 2.

Fig. 3a is a perspective view of the transverse field lens shown in Fig.3, together with a yoke.

Fig. 4 is a diagrammatical view of the arrangement of two transversefield lenses in an electron microscope according to the invention.

Fig. 4a is a cross sectional view of the diagrammatic arrangement shownin Fig. 4.

Referring to Fig. 1, the cross-hatched part represents a homogeneousmagnetic field, of which the lines of force are at right angles to theplane of the drawing. Two electron paths 1 and 2, located in this plane,enter the field under an angle of to the boundary plane 3 of themagnetic field. In the field they are circularly curved by the Lorenzforce and emerge from the field again as parallel lines. The circle arcsintersect with one another at point 4. If the boundary plane 3 is notflat, but curved with the axis 18 at point 19.

3 or broken in a manner such that the electrons in the field describe anare which diifers from 180, the initially parallel electron paths emergefrom the field at an angle to one another. It is thus possible toperform focussing, which only applies to electron paths in a plane atright angles to the lines of force.

If the pole surfaces between which the electron paths extend are notflat, but if they are shaped in the form of planes of revolution, inaccordance with the invention, a non-homogeneous magnetic field isproduced between these pole surfaces. If these pole surfaces extend overa sector of 270 or less, a beam of electrons may be tangentiallyintroduced into this field. The field has a focussing effect on thisbeam both in an axial and a radial direction. There is thus produced alens.

Referring to Fig. 2, 5 designates a pole shoe of such a transverse fieldlens, viewed in the direction of the axis of revolution 6. Fig. 3 showsthe two pole shoes 5 and 7 in a sectional view in the flat plane IIlllIthrough the axis 6 of Fig. 2. The sectional area of the pole surfaces 8and 9 is a parabola, of which the top 10 lies on the axis 6.

The two pole shoes are made of ferromagnetic material and are associatedwith one magnetic system. They are interconnected by means of a yoke 41(Fig. 3a.), which is surrounded by an energising winding or whichincludes a permanent magnet 42. If the system has a north pole at 7, thesouth pole is at 5. A non-homogeneous magnetic field prevails in thespace between these poles, through which a beam of energised particlesis passed. The strongly curved part of the pole surfaces near the axis6, extending beyond the optically operating range of the 'lens, is cutoff.

In the equatorial plane 11, which intersects the magnetic lines of forceat right angles, one of the particles, which enter the field in atangential direction, describes a circuit are about the axis 6. This areis to be considered as the optical axis of the lens. It is designated by12. Paths 13 and 14, which, previously to entering the lens, extendparallel to the optical axis 12 on either side thereof, are curved inthe magnetic field in a manner such that they intersect with the axis 12at point 15. This point, which lies at the boundary of the field in theexample shown, is the focal point of the lens.

Paths extending in the plane going through the optical axis 12 at rightangles to the equatorial plane and being parallel to one another beforethey enter the lens also curved in the non-homogeneous magnetic field ina manner such that they intersect with the axis 12 at point 15.Consequently complete focussing take place, i. e. about each point ofthe optical axis the focussing effect is the same in two directions atright angles to one another.

A focussing which is generally sufficient for practical purposes may beobtained by replacing the parabolic sectional areas of the pole surfacesby straight lines 16 and 17 i. e. by rendering the pole surfacesconical. The astigmatism produced by this approximation is minimized,if, in the area of the optical axis 12 the spacing between these linesis twice the spacing between their points 'of intersection with the axisof revolution 6, i. e. if a=2b. The straight lines 16 and ll7 aretangents to the parabolic lines 8 and 9 at points 34 and 35.

The position of the optical axis, i. e. the radius of the path in theequatorial plane, which is circularly curved,

'varies with the magnetic field strength and with the velocity of theparticles. If the voltage which accelerates the particles increases, theradius of this path also increases and the optical axis shifts inposition, for example, to occupy the circle are 18. The paths 13 and 14,which are both located on the same side of the new optical axis, arethen curved in a different manner, so that they intersect It followstherefrom that at a variation of the acceleration voltage the imageproduced by the transverse field lens shifts in position. With atransverse field lens having conical pole surfaces this variationfurthermore affects the image sharpness, since with this lensastigmatism is only absent, if the aforesaid condition with respect tothe position of the optical axis is fulfilled.

If the angle between the tangents or generatrices becomes large, thecurvature of the magnetic lines of force becomes so strong, that thelens will exhibit optical errors. It has been found that as long as theinclination of these lines remains below 30, these errors are, in mostcases, permissible.

The strength of the lens varies with the angle 0- between the endsurfaces 20 and 21 of the pole shoes, which may be termed the lensangle. The maximum lens strength is obtained, if this angle is 127%".The focal distance of a transverse field lens having a lens angle ofthis maximum value is R x/Z where R designates the radius of the are,which is formed by the optical axis of the lens. The pole shoe shown inFig. 2 has a lens angle 0 of the said optimum value, so that the focalpoints of the lens provided with such pole shoes are located in theboundary planes 2t) and 21. With a lens having conical pole surfacesthis only applies if the condition a=2b is fulfilled.

The maximum limit of 270 for the lens angle of the lens disclosed inthis application is set, since with angles exceeding 270 it is notalways possible to center the elec tron beam about the optical axis.

The property of an image producing lens, according to the invention,that at variation of the electron velocity, the image shifts inposition, i. e. the chromatic displacement, is a disadvantage for anelectron microscope provided with such a lens. If the lens should have aseparating power of A., the relative variation of the working voltagemay not exceed This requirement is severe and often difficult tofulfill. However, the chromatic displacement may be reduced and evencompletely obviated, if the electron beam emerging from the lens iscollected in a second transverse field lens, which curves the electronbeam in the same sense. At a variation of the working voltage adisplacement of the optical axis also occurs in this second lens, butowing to the reversal of the image, the displacement of the optical axisof the second lens corrects the displacement of the image. This isreadily comprehensible by considering how, with a system of two positiveglass lenses the course of the light rays varies when the system isdisplaced in a direction at right angles to the optical axis. For thispurpose the case is considered in which the first lens projects a real,reversed image in front of the focal point of the second lens, thisimage being reproduced again in a reversed manner by the latter.

By a suitable choice of the arrangement of the second lens, and, ifrequired, of one or more further lenses, the chromatic displacement maybe eliminated, at least as far as it is a lens error of the first order.The chromatic displacement which is left as a lens error of higher orderis so small that requirements for the constance of the working voltageand the magnetic field strength are less severe than with a longitudinalfield lens having the same separating power. A relative variation of ispermissible in many cases.

Figs 4 and 4a show an arrangement of two transverse field lenses for anelectron microscope according to the invention. These figures also showdiagrammatically the electron gun of the microscope. The latter isformed by a filament cathode 22, a Wehnelt cylinder 23 and an anode 24.The system formed by these electrodes emits a ray of electrons 25, ifsuitable voltages are applied thereto. This ray enters the transversefield lens 26 at right angles to one of the boundary surfaces designated1 by 20 and 21 in Fig. 2; the ray is curved therein in a manner suchthat it emerges from this lens in the direction 27. It is assumed thatthe axis of the electron ray coincides with the optical of the lens 26,which'may be ensured by a suitable choice of the voltage or of themagnetic field strength and a suitable arrangement of the lens. Thislens produces an amplified image at point 29 of a speciman arranged nearthe lens 26 at point 28. This image constitutes in its turn the objectfor a second transverse field lens 30, in which the ray is again curvedin the direction 31.

The lens 30 could be caused to produce an image on a collecting screenstruck by the ray 31. As an alternative, the microscope may be providedwith a third electronoptical system, if this is required for the controlof the amplification or for an increase in amplification.

The fact that the ray can be curved through a total angle of more than180 by the two lenses 26 and 30 provides the possibility of arrangingthe lenses in a manner such that the ray of electrons 31, emerging fromthe lens 30, enters into the electro-static field of the electrodesystem 23, 24. From an electron-optical point of view this system thenoperates as a convex mirror. If the image plane of this lens 30 liesbehind this mirror, for example, at 32, the mirror 23, 24 reproduces theimage on an enlarged scale in a plane located, for example, at 33'(seeFig. 4a). By means of a collecting screen (fluorescent screen orphotographic film) arranged in this plane this image may be renderedvisible. The mirror formed by the elements 23 and 24 (Fig. 4a) causesthe beam to curve around and strike the screen 33. The plane 32 (Fig. 4)indicates the plane in which an image would be formed if the mirror 23,24 was not functioning. With respect to the mirror 23, 24, therefore,the imaginary image formed in the plane 32 may be regarded as thevirtual object for producing the final image on the screen 33.

The ray 31 need not necessarily be exactly directed to the aperture ofthe anode 24 (Fig. 4), through which the electrons initially emerge. Asan alternative, at the side of the exit aperture of the anode for theray 25 (Fig. 4a), a second aperture may be provided for the ray 31,behind which is located the electrode 23, which is at a low negativepotential with respect to the electron source 22, when the microscopeoperates. Even this modified embodiment provides the advantage that themirror system does not require separate fastening means and supplyconductors.

The electron ray may be given its correct course by suitable choice ofthe distance of the lens 26 from the electron gun and of the spacingbetween the lenses 26 and 30. As an alternative, the ray emerging fromthe transverse field lens may be deflected in direction by varying thelens angle. An efiicient distance between the lens 26 and the electrongun is 50 centimeters. The spacing between the image plane 33 and theelectrode system 23, 24 may be of the same order.

The chromatic displacement is eliminated, if the following requirementis fulfilled:

R V 1 where V1 and V designate the amplifications produced by the lenses26 and 3t) respectively and R1 and R2 desigmate the radii of curvatureof the optical axes of the first and the second lens respectively. Ifthe two lenses are caused to produce the same amplification V, R2 mustbe equal to VRl. It has been found that practical results are obtainedby an amplification of each lens of approximately 7 and by anapproximately hundredfold amplification produced by the mirror 23, 24.This yields in total an amplification of about 5000, i. e., 7 7i00=4900, but owing to variations of the various factors affecting thetotal amplification, the result may be an amount which strongly divergestherefrom. A practical value of the radius of curvature R1 is, for

example, 3 mms. Then, in order to fulfill the aforesaid requirement ofelimination of the chromatic displacement, the radius of curvature R2must be 21 mms., if it is assumed that the two lenses produce asevenfold amplification.

The lenses 26 and 30 may be provided with a common yoke, so that a lenssystem is obtained which excells in simplicity and easy arrangement,particularly, if the lenses can be arranged closed to one another bychoosing the sum of the two lens angles to be only slightly in excess of180.

What I claimed is:

1. In an electron microscope including an electron beam source, andviewing means disposed in the path of the electron beam for producing avisible image therefrom, a magnetic lens for deflecting and focussingsaid beam on said viewing means comprising a pair of opposed spacedferromagnetic pole members defining tw opposed mirror-symmetrical polesurfaces each being a surface of revolution formed by a generatrixapproximating a part of a parabola the apex of which lies on the axis ofrevolution and shaped in the form of a sector of a circle having anangle of not less than and not more than 270, and a ferromagnetic yokeconnecting said pole members and forming therewith a closed magneticcircuit including an air-gap between the polemembers.

2. In an electron microscope including an electron beam source, andviewing means disposed in the path of the electron beam for producing avisible image therefrom, a magnetic lens for deflecting and focussingsaid beam on said viewing means comprising a pair of opposed spacedferromagnetic pole members defining two opposed mirror-symmetrical polesurfaces each being a conical surface of revolution formed by ageneratrix which is a tangent to a parabola the apex of which lies onthe axis of revolution and shaped in the form of a sector of a circlehaving an angle of not less than 90 and not more than 270, and aferromagnetic yoke connecting said pole members and forming therewith aclosed magnetic circuit including an air-gap between the pole-members.

3. In an electron microscope including an electron beam source, andviewing means disposed in the path of the electron beam for producing avisible image therefrom, a magnetic lens for deflecting and focussingsaid beam on said viewing means comprising a pair of opposed spacedferromagnetic pole members defining two opposed mirror-symmetrical polesurfaces each being a conical surface of revolution formed by ageneratrix which is a tangent to a parabola the apex of which lies onthe axis of revolution and shaped in the form of a sector of a circlehaving an angle of not less than and not more than and a ferromagneticyoke connecting said pole members and forming therewith a closedmagnetic circuit including an air-gap between the polemembers.

4. In an electron microscope including an electron beam source, andviewing means disposed in the path of the electron beam for producing avisible image therefrom, a magnetic lens for deflecting and focussingsaid beam on said viewing means comprising a pair of opposed spacedferromagnetic pole members defining two opposed mirror-symmetrical polesurfaces each being a conical surface of revolution formed by ageneratrix which is a tangent to a parabola the apex of which lies onthe axis of revolution and shaped in the form of a sector of a circlehaving an angle of about 127%. and a ferromagnetic yoke connecting saidpole members and forming therewith a closed magnetic circuit includingan air-gap between the pole-members.

5. In an electron microscope including an electron beam source, andviewing means disposed in the path of the electron beam for producing avisible image therefrom, a magnetic lens for deflecting and focussingsaid beam on said viewing means comprising a pair of opp sed. ,sp ed,ferro a net polemembersv defining two opposed mirron-symnietfical polesurfaces ;ea ch being. ,a conical sur face .of revolution formed by ageneratrix which is a tangent to a parabola the apex of whichlies onthevaxis-of revolution and shaped inthe form of a sector of a circlehaving an angle of not, less. than 90 and not more than 270,.and apermanentflmagnetyoke connectingv said pole, membersv and forming,therewith ,a closed magnetic'circuit including. an, air-gap between thepole-members.

6. In an electron microscope including an electron beam source, andviewing meanstdisposed in the path of the electron beam for producing ,avisible image therefrom, a magnetic lensSYStem for. deflecting andfocussing said beam on said viewing meanscomprising two pairs of opposedspaced ferromagnetic pole members successively positioned inthe pathof,theelectron beam, each pair of pole members defining two Opposedmirrorsymmetrical pole surfaces eachbeing a surface of revolution formedby a generatrix approximating a part of a parabola the apex of whichlieson the, axis of revolution and shaped in the form of a sector of acircle having an angle of not less than 90 and not more than 270, and aferromagnetic yoke connecting said pole members and forming therewith aclosed magnetic circuit including an air-gap between the pole-members.

7. In an electron microscope including an electron beam source having anacceleratingelectrode, and viewing means'disposed in the pathof theelectronbeam for producing a visible image therefrom, a magneticlensSYSIQmiQrUdefleQting andfocussingusaidbeam on s i view ugzmeanscomprising tw pairs of ppaomdsuace ferromagnetic lpole, memberssuccessively positioned in the pathoftheelectron beam, each pair oflpolemembers defining two opposed,mirror-symmetrical pole surfaces eachbeinga'surfacelof revolutionformed by a generatrixapproximating apart of aparabola the apex of which lieson the ,axis of revolution and shaped intheform of a sector of a circle having an angle of, not less than 90and, not more than 270", and aferromagnetic yoke connecting said pole,members andforming therewith a closed magnetic circuit including anair-gap between the polemembers, said respective pairs of pole-membersbeing further positioned to direct the electron beam toward saidaccelerating electrode whereby the latter jdefiects said beam towardsaid viewing means.

References Cited in theifile of this patent UNITED STATES PATENTSIHewitt Apr. 28, 1953

